1. Field of the Invention
The present invention relates to methods of manufacturing uranium oxide powder for use as nuclear fuel and, more particularly, to a two-step dry process for producing uranium oxide powder that eliminates the need for wet processing, and results in easy to handle UO2 powder and stable pellets.
2. Description of the Prior Art
The preparation of commercial nuclear fuels mainly has been by processes which use enriched and depleted uranium (i.e., enriched or depleted in the uranium-235 isotope compared to the uranium-235 content of naturally occurring uranium ore) feed as UF6. The enriched UF6 is converted to UO2 by processes selected to give the ceramic sinterability needed for the preparation of nuclear fuel pellets.
While procedures for converting UF6 to uranium oxides are known, currently available procedures are not particularly efficient or economical for converting UF6 to UO2. More specifically, the UF6 conversions for nuclear fuels have been developed to prepare UO2 with well controlled ceramic properties. Furthermore, because of the need to control their ceramic properties and because of thermodynamic limitations, the known commercial conversion processes are either complex aqueous-based processes with multiple process stages or a one-stage dry process. While the wet processes are easier to control, they produce large amounts of liquid wastes. The single step dry process produces a minimal waste stream but is difficult to operate.
Early patents issued to Reese et al., U.S. Pat. No. 3,168,369, filed in 1961, and to Blundell et al., U.S. Pat. No. 3,235,327, filed in 1962, described all the basic reactions and general technology required to make uranium dioxide nuclear fuel for nuclear reactors from uranium hexafluoride:

at 1000° F.-1800° F. or 537° C.-982° C. in dry processes. Here an inert gas could be used to promote a gas phase reaction between UF6 and H2O, as steam, to provide a very high surface area, uranyl fluoride (UO2F2) fluffy powder, having a tap density in the range 0.5 to 1.5 gm./cc. and a surface area in the range of 2 to 4 meters2/gm. (Tap density is obtained by putting the UO2 powder in a graduated cylinder and vibrating for a given time. This sets the volume and the graduated cylinder is weighed to obtain the weight of the powder.)
Numerous U.S. patents have been subsequently issued directed towards processes for the conversion of UF6 to uranium oxides. See, for example, U.S. Pat. No. 4,830,841 and the U.S. patents listed therein, which describe procedures for converting UF6 to uranium dioxide in furnaces, rotary kilns, fluidized beds and the like. For example, U.S. Pat. No. 4,830,841 is directed to a process for preparing UO2 from UF6 by reacting UF6 with steam to produce submicron uranyl fluoride powder, fluidizing a bed of uranium oxide material with a mixture of steam, hydrogen and inert gas at about 580° C. to about 700° C., and introducing the submicron uranyl fluoride powder into the fluidized bed of uranium oxide material so that the uranyl fluoride powder is agglomerated, densified, fluidized, defluorinated and reduced to a fluoride-containing uranium oxide material which is removed from the fluidized bed and then contacted with hydrogen and steam at elevated temperature to obtain UO2 essentially free of fluoride. The UO2 product produced from this process tends to be very inactive and requires an intense milling step to produce moderately active powder. In addition, there often is incomplete conversion of UO2F2 to UO3/U3O8, which leads to unacceptable contamination in the final UO2 powder. This likely is due to inadequate residence time and the growth of large particles in the initial phase which cannot complete the fluoride removal reaction. The differences between plural fluid bed reactors and flame reaction via flame plumes as used in this invention are dramatically, fundamentally different and nonequivalent, as discussed later.
Other U.S. patents disclose single-step processes for producing nuclear reactor fuel, such as U.S. Pat. No. 4,397,824 and U.S. Pat. No. 5,875,385. An exemplary single-step process for producing solid uranium oxide powder is disclosed in U.S. Pat. No. 5,752,158, which describes a single-step MDR (Modified Direct Route—this is really a trade name and not a very illustrative name) process for producing solid uranium oxide powder and gaseous HF from UF6 by bringing together two gaseous reactant streams, one of the streams comprising UF6 optionally admixed with oxygen as O2, and the second reactant stream comprising a mixture of hydrogen as H2 or as a hydrogen-containing compound and oxygen as an oxygen-containing compound. The gaseous reactant streams are brought together at a temperature and composition such that the UF6 is converted rapidly by flame reaction into readily separable solid uranium oxide and a gaseous HF product. Another single-step process is disclosed in U.S. Pat. No. 4,112,005, which describes reacting UF6 with steam within a first region of a vessel in which UO2F2 is obtained, which then is subjected to reduction within a second region of the vessel to obtain UO2. The UO2F2 obtained is contacted with a mixture of hydrogen gas and steam in a first zone of the second region of the vessel, in which an oxide having an intermediate composition between U3O8 and UO2 is contacted with the hydrogen gas and steam within a second zone of the second region of said vessel. The problem with these processes is the low feed rate due to the need to produce acceptable ceramic grade UO2 powder that can be made into dense UO2 pellets.
Additional single-step dry processes for obtaining uranium dioxide powder (i.e., by direct reduction of UF6 into UO2) which include the IDR (Intermediate Dry Route—another trade name, not very descriptive) process have been widely used and are described in, for example, U.S. Pat. No. 4,889,663; U.S. Pat. No. 4,397,824 and French No. 2,060,242. The powders obtained by the dry conversion process, including water vapor hydrolysis followed by pyrohydrolysis of the uranyl fluoride UO2F2 obtained, have the advantage of being readily sinterable. The powder produced is very active but hard to handle and produces very weak green pellets. Handling therefore is delicate and rejects are numerous if special care is not exercised. The IDR process converts UF6 to UO2 in a one-step, vapor/solid phase reaction that is hard to control and tends to produce a product with a UO2F2 contaminant. Part of the problem with this process is that two very exothermic processes occur in the same location at the tip of the mixing nozzle: (1) formation of UO2F2; and (2) some UO3/U3O8 from the reaction of steam and entrained hydrogen from the surrounding atmosphere. As the process flow rate is increased, the amount of hydrogen that is intermixed with the steam hydrolysis step becomes variable which produces large variations in the flame temperature and results in large variations in the powder properties.
There are several so-called double step processes, to produce UO2 from UF6, utilizing flame reactors and rotary kilns, connected by scroll/screw type rotatable moving means for the first reaction generated UO2F2 powder. The general problem with these processes is that step 1 production of UO2F2 is not, in fact, protected from H2 gas intrusion from step 2 formation of UO2 in a rotary kiln; and H2 intrusion into step 1 produces the variations in the powder properties described above. These seem not to be true commercial realizations, as H2 seepage through unfilled screw or scroll feeders leads to the reaction:UO2F2+N2+H2O+H2→UO2+2HF+H2O+N2,with uncontrolled temperature which produces either unreactive or too reactive powder.
Carter et al., U.S. Pat. No. 5,757,087, utilizes at least two obliquely positioned flame reactor plumes, to produce circulating product UO2F2 product powder, which is “scrolled” to a horizontal kiln for reaction with countercurrent flow of steam and/or H2 to provide UO2 through an outlet chamber. No example is given. Feugier, in U.S. Pat. No. 6,136,285, also utilizes a screw feeder between steps 1 and 2, and teaches concentric introduction of UF6 internally, and N2 in an annular space between UF6 and steam; to provide a reaction at a central nozzle tip injector in a flame reactor to provide UO2F2 and HF gas. The N2 is injected between the UF6 and the steam to keep crystalline UO2F2 from forming on the nozzle tip. All HF, excess steam and N2 gas must be exhausted through filters in the top of the flame rector, as they are the only HF off gas filters shown in the patent. While this patent concentrates on the concentric nozzle in the flame reactor, there seems little realization as to what happens to HF formed, and unreacted H2 and steam, in the second stage rotary pyrohydrolysis furnace which injects countercurrent steam and H2. This rotary furnace requires 5 zones, with its FIG. 8 showing kiln temperatures of over 680° C. in zones 1-4, with maximum temperatures of the interior gas of H2 HF and H2O of 730° C.-800° C., which temperatures should easily translate in a steady state process to the kiln shell. The only HF filters must also exit excess H2 and steam, passing through/bubbling through, under pressure, the screw progressing UO2F2 and exiting through the same set of filters as the off-gas from the flame reactor exits through. Only one set of filters is contemplated in the patent.
All of these processes provide substantial amounts of HF gas and micro entrained particles of UO2, UO2F2, and U3O8, which must be removed in order to make a by-product of HF, uncontaminated with any uranium compounds. One patent in particular, Feugier, U.S. Pat. No. 7,422,626, provides substantial detail in this area. There, again, filters are shown only in the stage 1 flame reactor, which seems to imply that HF and unreacted H2 and steam from the stage 2 rotary kiln pass through the screw transport to exit in the stage 1 flame reactor. Therefore, the flame reactor is not truly hydrogen free and as the flame transitions between laminar and turbulent flow and randomly entrains gases in the flame from the surrounding gas, random fluctuations in temperature occur leading to highly variable UO2F2 powder properties which leads to highly variable UO2 powder properties.
Here, as in all filter systems, filtering radioactive materials formed as fluffy particles must be cleaned by gas, such as N2 blowback. Feugier, U.S. Pat. No. 7,422,626, requires extremely radical sonic ejection of powder, by N2 blowback at speeds of over 300 m/s for less than 1 sec.; this is over about 700 mph (sonic speed=343.14 m/s at 20° C. which=707.58 mph). This is essential to their process. Sintered metal filters are well know, and almost all such filters have gas blowback valve means, as described in Mott Corporation Brochure, “Fiber Metal Gas Filtration” Rev. Feb. 10, 2008, and Union Carbide article by T. Shapiro et al. “Porous Metal Filters, Application to Feed Materials Production”, Jun. 15, 1961 (copy supplied to the British Library) where application of sintered porous metal filters to solid gas systems was operated in fluid bed systems. There, metal filters that have been plugged with dust, were cleaned by gas blowback at about 115 cubic feet/min., at 15 psi which translates to a velocity of about 417 feet/sec or 127 meters/sec which is below sonic speed. The calculation is as follows:
Nozzle openings= 3/16 inch
Number of nozzles=24 (page 15, FIG. 1)
Flow per nozzle=115/24=4.79 cubic feet/minute/nozzle
Velocity=4.79 ft3/(3/16.3/16.3.14/4 in2)*(144 in2/ft2)/(60 sec/min) (where * is equivalent to “times” or x=multiplied by)
Velocity=417 ft/sec or 127 meters/sec.
Ejectors are also shown in FIGS. 1, 4, 6, and 8 of T. Shapiro et al. (the small nubs over the opening of each filter in these figures) and described on page 12, 4th paragraph.
Another process for producing UO2 fuel pellets is disclosed in U.S. Pat. No. 5,091,120, which describes a method for producing fritted UO2 nuclear fuel pellets from metallic uranium. This method uses high value metal and therefore is not economically feasible.
U.S. Pat. No. 6,656,391 discloses the use of a wet ammonium diuranate process (ADU) to produce both UO3/U3O8 from both uranyl nitrate hexahydrate (UNH) and UF6. In particular, the UO3/U3O8 that is produced from this process then is processed in a calciner to produce UO2. The ADU process produces a stable but only moderately active (i.e., only achieves a final pellet density of about 97.5% on a consistent basis) UO2 powder. In addition, this process produces a large amount of liquid waste that must be treated to remove the fluoride. The common way of treatment is to add calcium hydroxide (Ca(OH)2) slurry which then forms a large amount of solids from the final neutralization of the fluoride as CaF2. Disposal of these solids is difficult due to their origin in a nuclear facility. The discharged liquid waste while having a very low residual fluoride, is still regulated and must be monitored for any discharge permits that are obtained. Furthermore, the nitrate-based recycle (UNH) must be spiked with HF in order for it to have reasonable handling properties during the centrifugation and drying steps and produces a significant amount of nitrate that must be handled in the discharge as well as fluoride. The nitrate disrupts the ammonia recovery process due to the required addition of sodium hydroxide to free the ammonia from the nitrate. Another problem is the carryover of NH4F in the dried UO3/U3O8 product to the final calciner. This fluoride tends to agglomerate the UO2 fines which reduces the overall powder activity and produces a semi-volatile NH4F material that plates out and plugs the off-gas vents of the calciner.
A further extended type fluid bed process for producing nuclear reactor fuels is disclosed in U.S. Pat. No. 4,053,559 (Hart et al.), which describes a three-step process using continuous, four stage fluidized beds interconnected in series to provide substantially complete conversion of UF6 to UO2. This process, however, is quite complicated, hard to operate and generates a UO2 product with much residual fluoride.
Notwithstanding the extensive prior efforts referred to above, there remains a substantial need for improved procedures for converting UF6 into solid UO2 that produces a highly active, ceramic grade UO2 powder at high production rates and which is easy to control, and which very importantly completely isolates steps where H2 reactant is completely excluded from initial first stage reactions, where it poses serious UO2 product variability problems. Use of fluid bed processes are not an answer due to the issues with forming un-reactive, large solids and residual fluoride removal.
It is, therefore, a major object of the invention to provide a block to H2 backflow into the first reaction stage, a calcination process that can produce UO2 with low residual fluoride levels and a product that has controlled particle sizes and a powder with good reactivity.
It is an object of the present invention to provide a two-step dry process for making nuclear grade, active UO2 powder which tightly controls the exothermicity of the process steps and thus allows for very tight temperature control of each process step, and allows dual HF gas filtering, and particulate recirculation of entrained particles in the off-gases.
It is a further object of the present invention to provide a two-stage process wherein UF6 first is converted to UO2F2 using steam and then converting the UO2F2 to UO2 using a mixture of steam and hydrogen, which UO2 contains only very small amounts of unconverted UO2F2 (less than about 50 ppm).
It is a further object of the present invention to provide a two-stage process for making nuclear grade, stable, active UO2 powder, in which the two-stage process is carried out in two kilns, calciners or in flame reactors in which significant amounts of solids are retained in the kiln or calciner or are entrained in the flame reactor flame.